Filament diameter doesn't affect G Code

Hi. I am using Slic3r 1.2.9 and I have realized that when I the advanced Printer Settings option "Use velocity extrusion" the filament diameter doesn't affect the output G-code.
The description of this setting is the following:
" This experimental setting uses additional M600 commands to set the cross area of a printed line. Instead of generating the movement of the extruder per extruder axis, the movement of the extruder is calculated by ther firmware based on the velocity of the printing nozzle."

So how can the printer tell apart what filament diameter I am using? Is it included directly into the firmware?